1. Field of the Invention
In addition, by increasing on The present invention relates to a microstrip transmission line structure with vertical stubs for reducing far-end crosstalk, and more particularly, to a microstrip transmission line structure capable of reducing far-end crosstalk that occurs due to an electromagnetic coupling between adjacent transmission lines when several high-speed signals are transmitted through a microstrip transmission line.
According to the present invention, vertical stub structures for increasing a mutual capacitance are added to microstrip line transmission lines to reduce far-end crosstalk. Accordingly, without using a guard trace for a high-speed system having a limited area of a printed circuit board or increasing a distance between two signal lines, far-end crosstalk can be effectively reduced, so that the area of the printed circuit board can be decreased, and costs can be reduced.
In addition, by increasing only the mutual capacitance while maintaining a mutual inductance, jitter that occurs due to a difference between transmission times in the even and odd modes can be reduced, so that a signal transmission speed can be increased.
2. Description of the Related Art
Far-end crosstalk is caused by an electromagnetic coupling between signal lines and may generate timing jitter when high-speed signals are transmitted, so that the far-end crosstalk becomes a problem with increasing a signal rate. The Far-end crosstalk occurs due to a difference between a capacitive coupling caused by a mutual capacitance and an inductive coupling caused by a mutual inductance.
FIG. 1 is a view illustrating a conventional microstrip transmission line structure. In FIG. 1, two parallel microstrip transmission lines are illustrated. Ends of the transmission lines are terminated with resistors having the same value as a characteristic impedance.
The transmission line having an end (transmitting end) applied with a signal is referred to as an aggressor line 10, and the transmission line having an end that is not applied with a signal is referred to as a victim line 20. Far-end crosstalk VFEXT of the victim line 20 may be represented by Equation 1.
                                          V            FEXT                    ⁡                      (            t            )                          =                              TD            2                    ·                      (                                                            C                  m                                                  C                  T                                            -                                                L                  m                                                  L                  S                                                      )                    ·                                    ∂                                                V                  a                                ⁡                                  (                                      t                    -                    TD                                    )                                                                    ∂              t                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Here, TD denotes a transmission time for which a signal is transmitted along a transmission line, Cm denotes a mutual capacitance per unit length, CT denotes a sum of a self-capacitance and the mutual capacitance per unit length, Lm denotes a mutual inductance per unit length, LS denotes a self-inductance per unit length, and Va(t) denotes a voltage applied to a transmitting end of the aggressor line.
In a transmission line disposed in a homogeneous medium such as a stripline, the capacitive coupling and the inductive coupling have the same value, so that ideally, far-end crosstalk becomes 0.
However, in a microstrip line manufactured on a printed circuit board, the inductive coupling is greater than the capacitive coupling, so that the far-end crosstalk has a negative value.
The far-end crosstalk of the stripline transmission line can be removed. However, to do this, the stripline transmission line uses a larger number of layers of the printed circuit board as compared with the microstrip line, and this requires additional costs.
When individual signals are applied to the two parallel microstrip lines, a case where the two applied signals are changed in the same direction with respect to time is called an even mode, and a case where the two applied signals are changed in the opposite directions to each other with respect time is called an odd mode.
FIG. 2 is a conceptual diagram of the even mode and the odd mode. When an applied signal increases with respect to time, the far-end crosstalk has a negative pulse. Therefore, in the even mode, the far-end crosstalk delays the change in the signal with respect to the time, and in the odd mode, the far-end crosstalk reinforces the change in the signal with respect to time.
Therefore, a signal transmission time is slightly increased in the even mode and slightly decreased in the odd mode. A difference between the transmission times of the even and the odd modes may be represented by Equation 2 as follows.
                                          TD            EVEN                    -                      TD            ODD                          =                  l          ⁢                                                    L                S                            ⁢                              C                T                                              ⁢                      (                                                            L                  m                                                  L                  S                                            -                                                C                  m                                                  C                  T                                                      )                                              [                  Equation          ⁢                                          ⁢          2                ]            
Here, l denotes a length of the transmission line, TDEVEN denotes the even mode transmission time, TDODD denotes the transmission time in the odd mode, Cm denotes a mutual capacitance per unit length, CT denotes a sum of a self-capacitance and the mutual capacitance per unit length, Lm denotes a mutual inductance per unit length, and LS is a self-inductance per unit length.
When random data signals are applied to transmitting ends of two parallel microstrip transmission lines, due to a difference between signal arrival times in the even and the odd modes, times at which the data signals rise are different at receiving end. In other words, timing jitter occurs. This phenomenon is illustrated by dotted lines in FIG. 3.
In order to reduce the far-end crosstalk effects in the microstrip transmission line, distances between signal lines are increased, or guard traces are used. The guard trace is referred to as a structure in which a parallel trace is added between adjacent two signal lines to reduce a coupling between the two signal lines. However, the aforementioned methods require large areas of the printed circuit board.